It is known that a MTJ (Magnetic Tunnel Junction) element serving as a magnetoresistive element has a stacked structure as a basic structure, and shows a tunneling magnetoresistive (TMR) effect. The stacked structure is formed by a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer. Such MTJ elements are used in 100 bits per square inch (Mbpsi) HDD heads and magnetic random access memories (MRAMs).
A MRAM characteristically stores information (“1”, “0”) depending on changes in the relative angle of the magnetizations of magnetic layers included in each MTJ element, and therefore, is nonvolatile. Since a magnetization switching takes only several nanoseconds, high-speed data writing and high-speed data reading can be performed. Therefore, MRAMs are expected to be the next-generation high-speed nonvolatile memories. If a technique called a spin torque transfer switching technique for controlling magnetization through spin-polarized current is used, the current density can be made higher when the cell size of the MRAM is made smaller. Accordingly, high-density, low-power-consumption MRAMs that can readily invert the magnetization of a magnetic material can be formed.
Furthermore, in recent years, attention has been drawn to the theory that a magnetoresistance ratio as high as 1000% can be achieved by using MgO as the tunnel barrier layer. Since the MgO is crystallized, selective tunneling conduction of the electrons having a certain wavenumber from the ferromagnetic layer can be performed, and those electrons can keep the wavenumber during that time. At this point, the spin polarization has a large value in a certain crystalline orientation, and therefore, a giant magnetoresistive effect appears. Accordingly, an increase in the magnetoresistive effect of each MTJ element leads directly to a higher-density MRAM that consumes less power.
Meanwhile, to form high-density nonvolatile memories, high integration of magnetoresistive elements is essential. However, the ferromagnetic bodies forming magnetoresistive elements have thermal disturbance resistance that is degraded with a decrease in element size. Therefore, how to improve the magnetic anisotropy and thermal disturbance resistance of each ferromagnetic material is a critical issue.
To counter this problem, trial MRAMs using perpendicular-magnetization MTJ elements in which ferromagnetic bodies have magnetization directions perpendicular to the film plane have been made in recent years. In a perpendicular-magnetization MTJ element, a material having high magnetic crystalline anisotropy is normally used for ferromagnetic bodies. Such a material has a magnetization in a certain crystal direction, and the magnetic crystalline anisotropy of the material can be controlled by changing the composition ratio between constituent elements and the crystallinity of the constituent elements. Accordingly, the magnetization direction can be controlled by changing the direction of crystal growth. Also, since ferromagnetic bodies have high magnetic crystalline anisotropy, the aspect ratio of the element can be adjusted. Furthermore, having high thermal disturbance resistance, ferromagnetic bodies are suited for integration. In view of those facts, to realize a highly-integrated MRAM that consumes less power, it is critical to manufacture perpendicular-magnetization MTJ elements that have a great magnetoresistive effect.